Content delivery system

ABSTRACT

The server device  100  transmits media data of a media rate, calculated by the rate output means  102 , to the client device  200  at a transmission rate calculated by the rate output means  102 . The rate output means  102  uses an objective function, which uses a media rate and a transmission rate as variables and determines an evaluation value evaluating a combination of QoE and a delivery cost so as to correspond to the variables, and predetermined constrains, to calculate a combination of a media rate and a transmission rate with which the evaluation value is the highest. The client device  200  reproduces content based on the media data transmitted from the server device  100.

This application is the National Phase of PCT/JP2012/007372, filed Nov.16, 2012, which claims priority to Japanese Application No. 2011-279213,filed Dec. 21, 2011, the disclosures of which are hereby incorporated byreference in their entirety.

TECHNICAL FIELD

The present invention relates to a content delivery system fordelivering content, and in particular, to a content delivery systemallowing optimum content delivery while considering users' QoE (Qualityof Experience) and a delivery cost.

BACKGROUND ART

Along with the development of broadband networks and high-performanceinformation terminals such as mobile telephones and PCs, services ofdelivering content such as large volume videos and music over networksare increasing.

In a video delivery service, when videos of SD (Standard Definition)quality are transmitted, a large quantity of data communications ofabout 6 Mbps (data quantity of 6 megabits per second) are required forMPEG-2 (Motion Picture Expert Group-2). If any data loss orcommunication delay occurs during communications, videos may bedistorted or video reproduction may be stopped due to delayed arrival ofvideos, which degrades the user's QoE. As such, a conventional videodelivery service has been performed in such a manner that acommunication provider secures a large-capacity band in order to ensureQoS (Quality of Service) so as to prevent data loss from arising on theway.

However, along with the widespread use of broadband Internet in recentyears, video delivery services over networks, in which QoS is notassured such as the Internet, are increasing. The Internet hascharacteristics that an available network band varies depending oncommunications of other users. Further, higher-speed mobile networksbrought about by LTE (Long Term Evolution) or the like andhigher-performance terminals resulting from widespread use ofsmartphones are outstanding. As such, needs for viewing videos usingmobile networks are increasing. Mobile networks have characteristicsthat a network band varies widely depending on, in addition tocommunications of other users, variations in the radio field intensitybetween a base station and a user terminal caused by the movement of theuser, and effects of other wireless devices.

A delivery service for transmitting media data (data representingcontent) over such networks, in which QoS is not assured, is disclosedin Patent Document 1.

In the case of transmitting media data over a network in which QoS isnot assured, when the band which is available in the network (availableband) is narrowed, the entire media data cannot be transmitted. As such,videos may be distorted due to a packet loss, or reproduction of mediadata may be stopped due to delayed arrival of packets. Distortion ofmedia data and stop of reproduction of media data cause userdissatisfaction, whereby the user's QoE is degraded.

In order to cope with such a problem, Patent Document 1 discloses atechnology which uses a transmission rate and a media rate, whentransmitting media data from a server device to a client device, asvariables, and calculates a combination of a transmission rate and amedia rate with which the QoE becomes the highest, based on an objectivefunction associating those variables with QoE, and predeterminedconstraints. More specifically, among combinations of media rates andtransmission rates satisfying the constraints, a combination of a mediarate and a transmission rate with which the value of an objectivefunction becomes the smallest is calculated. Then, media data of thecalculated media rate is transmitted from the server device to theclient device at the calculated transmission rate. Here, a transmissionrate is a quantity of media data which is transmitted per unit time, anda media rate is a quantity of media data which is required forreproducing content for a unit time at a constant speed.

In the above-described objective function, the value thereof becomessmaller as the transmission rate is lower or the media rate is higher.Further, the above-described constraints include a limitation that atransmission rate does not exceed an available band and a condition thata media rate is any one of predetermined media rates.

By setting an objective function such that the value thereof becomessmaller as the transmission rate is lower, a lower transmission rate isselected preferentially, and a probability that congestion is caused inthe network can be lowered, so that occurrence of a packet loss can bereduced. Further, by setting an objective function such that the valuethereof becomes smaller as the media rate is higher, a higher media rateis selected preferentially, so that the content quality can be improved.As such, the technique described in Patent Document 1 is able to improveQoE.

Patent Document 1: WO 2011/018868

SUMMARY

The technology described in Patent Document 1 is able to improve QoE.However, as the technology described in Patent Document 1 merely selectsa combination of a transmission rate and a media rate capable ofimproving QoE (a combination of a transmission rate and a media ratewhich satisfy constrains and allow the value of an objective function tobe the smallest) as much as possible, there is a problem that it isimpossible to make the cost for delivering content (delivery cost)appropriate.

For example, in the case of reproducing content by streaming, in orderto sequentially reproduce content seamlessly even if the traffic or thelike of the network varies, it is necessary to store media data nothaving been reproduced for a predetermined time period in the clientdevice. Meanwhile, the user of the client device is free to stop viewingof the content, and if the user stops viewing in the middle of thecontent, the media data stored in the client device and not having beenreproduced is discarded. As such, if the user stops viewing in themiddle of the content, it means that the delivery cost was used todeliver useless media data. In the technology described in PatentDocument 1, as media data of a high media rate is preferentiallytransmitted from a server device to a client device, a larger quantityof media data not having been reproduced is stored in the client device.As such, if the user stops viewing in the middle of the content, itmeans that a larger delivery cost was used for delivering a largerquantity of useless media data.

As such, an object of the present invention is to provide a deliverysystem in which a problem that it is impossible to make the deliverycost for delivering content appropriate, if QoE is improved, is solved.

A content delivery system, according to an aspect of the presentinvention, is a content delivery system including a server device and aclient device which are adapted to be communicable with each other,wherein

the server device is adapted to be able to transmit media data, formedby encoding content, to the client device, and

the client device is adapted to, while receiving the media datatransmitted by the server device, store received data of the media datain a media reproduction buffer, read stored media data from the mediareproduction buffer, and reproduce the content.

The content delivery system further includes

a rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables; and

a media data transmission means for transmitting the media data, encodedat the media rate output from the rate output means, from the serverdevice to the client device at the transmission rate output from therate output means.

A server device, according to another aspect of the present invention,is a server device adapted to be communicable with a client device andto be able to transmit media data, formed by encoding content, to theclient device.

The server device includes

a rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables; and

a media data transmission means for transmitting the media data, encodedat the media rate output from the rate output means, from the serverdevice to the client device at the transmission rate output from therate output means.

A client device, according to another aspect of the present invention,is a client device adapted to be communicable with a server device and,while receiving media data transmitted by the server device, storereceived data of the media data in a media reproduction buffer, readstored media data from the media reproduction buffer, and reproducecontent.

The client device includes

a rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables.

A delivery method, according to another aspect of the present invention,is a delivery method to be applied to a content delivery systemincluding a server device and a client device which are adapted to becommunicable with each other. The server device is adapted to be able totransmit media data, formed by encoding content, to the client device,and the client device is adapted to, while receiving the media datatransmitted by the server device, store received data of the media datain a media reproduction buffer, read stored media data from the mediareproduction buffer, and reproduce the content.

The delivery method includes

calculating and outputting a combination of a transmission rate and amedia rate based on an objective function, the transmission raterepresenting a quantity of the media data transmitted per unit time bythe server device, the media rate representing a quantity of the mediadata per unit time, the objective function using the transmission rateand the media rate as variables and determining an evaluation valueevaluating a combination of QoE (Quality of Experience), which is aservice quality that a user of the client device experiences whenviewing the content reproduced by the client device, and a delivery costrepresenting a cost involved in delivering the media data so as tocorrespond to the variables; and

transmitting the media data, encoded at the output media rate, from theserver device to the client device at the output transmission rate.

According to the present invention, it is possible to achieve anadvantageous effect of improving QoE while reducing the delivery cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the schematic configuration of acontent delivery system according to a first exemplary embodiment of thepresent invention.

FIG. 2 is a block diagram showing functions of the content deliverysystem according to the first exemplary embodiment of the presentinvention.

FIG. 3 is a flowchart showing exemplary processing of a rate outputmeans 102 according to the first exemplary embodiment of the presentinvention.

FIG. 4 is a block diagram showing functions of a content delivery systemaccording to a second exemplary embodiment of the present invention.

FIG. 5 is a block diagram showing the schematic configuration of acontent delivery system according to a third exemplary embodiment of thepresent invention.

FIG. 6 is a block diagram showing functions of the content deliverysystem according to the third exemplary embodiment of the presentinvention.

FIG. 7 is a block diagram showing functions of a content delivery systemaccording to a fourth exemplary embodiment of the present invention.

EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the drawings.

First Exemplary Embodiment

As shown in FIG. 1, a content delivery system according to a firstexemplary embodiment of the present invention includes a server device100 and a client device 200. The server device 100 and the client device200 are communicably connected with each other over a network (NW)(e.g., Internet, carrier network, or mobile network).

The server device 100 includes a central processing unit (CPU) andstoring units (memory and HDD (Hard Disk Drive)), not shown. The serverdevice 100 is adapted to realize the functions described below by meansof execution of a program, stored in the storing unit, by the CPU.

The client device 200 is a personal computer. It should be noted thatthe client device 200 may be a mobile telephone, a PHS (PersonalHandy-phone System), a PDA (Personal Data Assistant), a smartphone, atablet terminal, a TV receiver, a set top box, a game console, a carnavigation device, or the like.

The client device 200 includes a central processing unit (CPU), astoring unit (memory), input devices (e.g., keyboard, mouse, and thelike), and output devices (e.g., display, speaker, and the like), notshown. The client device 200 is adapted to realize the functionsdescribed below by means of execution of a program, stored in thestoring unit, by the CPU.

FIG. 2 is a block diagram showing functions of a distribution systemconfigured as described above. These functions are realized by means ofexecution of a program shown by the flowchart of FIG. 3, to be describedbelow, and the like, by the CPU of the server device 100, and by meansof execution of a predetermined program by the client device 200.

[Configuration of Server Device 100]

The server device 100 includes a media data storing means 101, a rateoutput means 102, a media packet generation means 103, and a media datatransmission means 104. The rate output means 102 includes an optimumrate calculation means 102 a, a QoE (Quality Experience) evaluationmeans 102 b, and a delivery cost evaluation means 102 c.

The respective means of the server device 100 operate as describedbelow.

The media data storing means 101 stores one or more pieces of mediadata. Media data is data formed by encoding content (including mediasuch as one or more pieces of video data, audio data, subtitles data,and the like). In general, each medium is compressed by a videocompression method or an audio compression method in order to compressthe quantity of data, and encoded. In the present embodiment, withrespect to one content, a plurality of pieces of media data of differentmedia rates are stored. A media rate is a quantity of data (data size,that is, the number of bytes or bits, for example) per unit time ofencoded content. In other words, a media rate is a quantity of mediadata required for reproducing content for a unit time at a constantspeed (reproducing at a single speed).

The rate output means 102 outputs a transmission rate and a media ratecorresponding to a state of the server device 100, a state of the clientdevice 200, a state of a network NW linking the server device 100 andthe client device 200, and the like. A transmission rate is a quantityof data (data size, e.g., the number of bytes) to be transmitted by theserver device 100 to the client device 200 per unit time (e.g., onesecond). The details of the rate output means 102 will be describedbelow.

The media packet generation means 103 acquires media data whichcorresponds to the content to which a delivery request is made from theclient device 200, and is encoded at a media rate output from the rateoutput means 102. Then, in order to transmit the acquired media data tothe client device 200, the media packet generation means 103 generatesmedia packets.

The media data transmission means 104 outputs the media packets,generated by the media packet generation means 103, to the client device200 in such a manner that a quantity of data transmitted per unit timeconforms with the transmission rate output from the rate output means102. In the present embodiment, it is assumed that a content file in 3GPfile format provided by 3GPP, in MP4 file format provided by ISO/IEC, inAVI format, or the like, is divided into packets, and transmitted usingHTTP (Hyper Text Transfer Protocol) or TCP/IP. However, the presentinvention is not limited to these examples. It is also possible togenerate and transmit media packets using another method or protocol,such as transmitting them using RTP (Real-Time Transport Protocol) orUDP/IP.

[Configuration of Client Device 200]

The client device 200 includes a media data reception means 201, a mediareproduction buffer 202, a media data reproduction means 203, areception information acquisition means 204, a reproduction speedacquisition means 205, and a remaining reproduction time acquisitionmeans 206.

The respective means of the client device 200 operate as describedbelow.

The media data reception means 201 receives media packets transmittedfrom the server device 100, and stores the media data, included therein,in the media reproduction buffer 202.

The media data reproduction means 203 reads and reproduces the mediadata stored in the media reproduction buffer 202, and presents them tothe user via an output device not shown. The media data reproductionmeans 203 is also able to change the reproduction speed in accordancewith an input of an instruction relating to the reproduction speed suchas constant-speed reproduction, slow reproduction, double-speedreproduction, or rewind, which is input via an input device not shown.

This means that as the client device 200 is adapted to, while storingmedia data from the server device 100, received by the media datareception means 201, in the media reproduction buffer 202, reproducemedia data which is stored in the media reproduction buffer 202 and hasnot been reproduced, by the media data reproduction means 203, theclient device 200 is able to reproduce media data before the entire datahas been downloaded. As such, the client device 200 is able to reproducemedia data by means of streaming or progressive download. If, duringreproduction of media data by means of streaming or progressivedownload, media data not having been reproduced (media data to bereproduced) becomes empty, reproduction of the media data by the mediadata reproduction means 203 will be stopped. If reproduction of themedia data is stopped, the user feels uncomfortable, so that QoE isdegraded.

The reception information acquisition means 204 transmits a valuecalculated by dividing the quantity of the media data, sequentiallyreceived by the media data reception means 201, by the reception timeperiod of the media data, to the server device 100 as receptioninformation. For example, if the media data reception means 201sequentially receives media data of “0.5 megabytes (MB)” in “0.5seconds” and then cannot receive media data for a given time period, thereception information is [0.5 MB/0.5 seconds=1 (MB/s)]. The calculationprocessing of the reception information is performed every predeterminedperiod. While it is assumed that media data is received using TCP/IP inthe media data reception means 201, it is also possible to use UDP/IP.In the case of using UDP/IP, there is a possibility that media packetstransmitted from the server device 100 are lost and cannot be receivedby the media data reception means 201. In that case, a rate of loss ofmedia packets (rate of occurrence of packet loss) is also acquired andtransmitted to the server device 100. For example, in the case of usingRTP, a rate of packet loss is calculated by specifying the lost mediapackets based on the sequence number included in the RTP header.

The reproduction speed acquisition means 205 acquires a reproductionspeed of media data reproduced by the media data reproduction means 203and transmits it to the server device 100. A reproduction speed is avalue obtained by converting the quantity of media data reproduced perunit time by the media data reproduction means 203 into a ratio to thequantity of data to be reproduced per unit time in constant-speedreproduction. In other words, a reproduction speed is a value obtainedby dividing the quantity of media data reproduced per unit time by themedia data reproduction means 203 by the quantity of media data to bereproduced per unit time in constant-speed reproduction. For example, ifdata for 1 second in media data is reproduced in 1 second, thereproduction speed is “1”, which corresponds to the case where the userreproduces it at a constant speed without using fast-forward, slowreproduction, rewind, or the like. If the data is fast forwarded at adouble reproduction speed, the reproduction speed is “2”, if reproducedslowly at a half speed, the reproduction speed is “0.5”, while ifinversely reproduced at a constant speed, the reproduction speed is“−1”.

The remaining reproduction time acquisition means 206 acquires a periodof time during which content is able to be reproduced at a constantspeed (remaining reproduction time) by the media data reproduction means203, based on the portion not having been reproduced (non-reproduceddata) of the media data stored in the media reproduction buffer 202, andtransmits it to the server device 100. For example, if data for 30seconds from the beginning of the media data is stored in the mediareproduction buffer 202 and data for 20 seconds has been reproduced bythe media data reproduction means 203, the remaining reproduction timeis 10 seconds (=30 seconds−20 seconds).

In the present embodiment, while it is assumed that the receptioninformation acquisition means 204, the reproduction speed acquisitionmeans 205, and the remaining reproduction time acquisition means 206transmit data from the client device 200 to the server device 100 usingHTTP, it is also possible to transmit data using RTCP (RTP ControlProtocol) if RTP is used, or using other protocols.

[Description of Operation of First Exemplary Embodiment]

Next, operation of the present embodiment will be described in detailwith reference to the block diagram of FIG. 2 and the flowchart of FIG.3.

When a user selects content that the user wishes to view in the clientdevice 200 and makes a delivery request for media data from the clientdevice 200 to the service device 100, delivery of the media data begins.At the start of delivering the media data, the server device 100transmits the media data of a predetermined media rate, at apredetermined transmission rate.

When delivery of the media data begins, the rate output means 102acquires the state of the server device 100, the state of the clientdevice 200, and the state of the network NW (step S001 in FIG. 3).

[State of Client Device 200]

In the present embodiment, a state of the client device 200 includes areproduction speed of the media data transmitted from the reproductionspeed acquisition means 205, and a remaining reproduction time of themedia reproduction buffer 202 transmitted from the remainingreproduction time acquisition means 206. A state of the client device200 may also include types and protocols of media which can bereproduced by the client device 200, and a CPU load and available memoryof the client device 200.

The state of the client device 200 transmitted from the reproductionspeed acquisition means 205 and the remaining reproduction timeacquisition means 206 in the client device 200 is input to the rateoutput means 102 in the server device, over the network NW.

While the present embodiment is configured such that a reproductionspeed is transmitted from the reproduction speed acquisition means 205to the server device 100, it is also possible to transmit the content ofan instruction (constant-speed reproduction, slow reproduction, rewind,or the like) relating to the reproduction input by the user of theclient device 200. The rate output means 102 is able to estimate thecurrent reproduction speed based on the content of the instructiontransmitted from the client device 200. For example, if the content ofthe instruction is “constant-speed reproduction”, the rate output means102 estimates a reproduction speed “1”, while if the content of theinstruction is “slow reproduction”, the rate output means 102 estimatesa reproduction speed “0.5”. It is also possible that with a restrictionof not allowing trick play such as fast-forward, slow reproduction, andrewind by the client device, the reproduction speed can be fixed to “1”.In that case, the reproduction speed acquisition means 205 isunnecessary.

Further, although the present embodiment is configured such that aremaining reproduction time is transmitted from the remainingreproduction time acquisition means 206 to the server device 100, it isalso possible to estimate a remaining reproduction time by the rateoutput means 102 in the following manner, without transmitting aremaining reproduction ti from the remaining reproduction timeacquisition means 206.

In general, at the start of reproducing media data, reproduction of themedia data is not started until a certain quantity of media data isstored in the media reproduction buffer 202. Here, if a quantity ofmedia data which must be stored in the media reproduction buffer 202(initial buffer quantity) in order to start reproduction is Bi(seconds), a quantity of media data having been transmitted from theserver device 100 during a time period t is dM(t) (bytes), a media rateof the media data is rM(t) (bytes/second), and a reproduction speed inthe client device 200 in the time period t is p(t), a remainingreproduction time br(t) (seconds) is represented as Expression (1) shownbelow. However, until the initial buffer quantity Bi of the media datais stored in the reproduction buffer 202, the reproduction speed p(t)=0.br(t)=(∫dM(t)/rM(t)−p(t))dt  (1)

Here, it is assumed that the initial buffer quantity Bi is 5 seconds,the reproduction speed p(t) in the client device 200 is 1, and the mediadata, in which the media rate rM(t) is 1 Mbps (1 megabit (=125kilobytes) per second), is transmitted 2 megabytes in 10 seconds fromthe server device 100 to the client device 200.

In this case, as the transmission rate is [2 megabytes (Mbytes)/10seconds=0.2 (Mbytes/second)], the time required for storing the mediadata for the initial buffer quantity Bi=5 seconds in the mediareproduction buffer 202 is [0.125 (Mbytes/second)*5 (seconds)/0.2(Mbytes/second)=3.125 (seconds)]. Accordingly, the remainingreproduction time br (10) is calculated to be 9.125 (seconds) as shownby the following Expression (2).

$\begin{matrix}\begin{matrix}{{{br}(10)} = {{2{({Mbytes})/0.125}\left( {{Mbytes}\text{/}{second}} \right)} -}} \\{\left( {{10\mspace{14mu}{seconds}} - {3.125\mspace{14mu}{seconds}}} \right)} \\{= {{16({seconds})} - {6.875({seconds})}}} \\{= {9.125({seconds})}}\end{matrix} & (2)\end{matrix}$

Further, in the case where the user of the client device 200 views themedia data at a constant speed for 4 seconds from the time when the userstarted reproduction of the media data, and then views it at a doublespeed, the remaining reproduction time br (10) is 3.125 (seconds) asshown by the following Expression (3).

$\begin{matrix}\begin{matrix}{{br} = {{2{({Mbytes})/0.125}\left( {{Mbytes}\text{/}{second}} \right)} -}} \\{\left\{ {{4\mspace{14mu}{seconds}} + {\left( {{6\mspace{14mu}{seconds}} - {3.125\mspace{14mu}{seconds}}} \right)*2}} \right\}} \\{= {{16({seconds})} - {9.75({seconds})}}} \\{= {6.25({seconds})}}\end{matrix} & (3)\end{matrix}$

Here, the quantity of data and the media rate of the media data havingbeen transmitted from the server device 100 to the client device 200 canbe measured by the server device 100. As such, if the server device 100is able to know the reproduction speed p(t) in the client device 200, itis possible to calculate the remaining reproduction time by the rateoutput means 102, without the remaining reproduction time acquisitionmeans 206. In particular, with a restriction of not allowing trick playsuch as fast-forward, slow reproduction, and rewind by the client device200, it is possible to calculate the remaining reproduction time br(t)by fixing the reproduction speed p(t) to “1”.

[State of Server Device 100]

A state of the server device 100 may include the number of clientdevices 200 to which content is delivered simultaneously by the serverdevice 100, a CPU load, available memory, the entire quantity ofcommunications transmitted from the server device 100, a data transferquantity involved in reading of the media data from the media datastoring means 101, and the like. These kinds of information can beacquired using the well-known techniques.

[State of Network NW]

A state of the network NW includes a bandwidth of available band whichcan be used for transmitting media data from the server device 100 tothe client device 200 (which may be simply referred to as availableband), and a rate of packets which did not reach the client device 200because of a packet loss on the network (packet loss rate). The statemay also include a rate of congestion of the entire network.

While, in the present embodiment, it is assumed that receptioninformation (value obtained by dividing the quantity of media datasequentially received by the media data reception means 201, by the timeperiod of receiving the media data) transmitted from the receptioninformation acquisition means 204 is handled as an available band, anavailable band can be calculated in the following manner.

If it is possible to estimate a packet loss rate or it is known thatthere is no packet loss, an available bandwidth can be calculated bysubtracting the quantity of data of packet loss from the quantity ofdata transmitted from the media data transmission means 104 of theserver device 100, so that the reception information acquisition means204 may be unnecessary. In particular, in the case of transmitting mediadata using TCP/IP, the lost packets are transmitted again due to thecharacteristics of TCP/IP so that a packet loss will not occur. As such,an available band can be measured easily from the quantity oftransmitted data. In that case, communications from the receptioninformation acquisition means 204 is unnecessary.

Further, it is also possible to acquire an available band bytransmitting prove data, which is different from the media data, to theclient device 200. As this kind of method, packet train estimationmethods such as PathLoad, pathChirp, and the like have been known. It ispossible to calculate an available band by means of a packet trainestimation method without using the reception information acquisitionmeans 204.

The processing performed at step S001 is as described above.

When the processing of step S001 is completed, the rate output means 102evaluates the user's QoE and the delivery cost from the states of theserver device 100, the client device 200, and the network NW, andcalculates a transmission rate and a media rate which are optimum forthe media data to be delivered next (step S002 in FIG. 3).

[Evaluation of QoE]

QoE is evaluated by the QoE evaluation means 102 b of the rate outputmeans 102. The grounds for degradation of QoE, at the time of viewingcontent, are as follows.

(a) Packet loss. When a packet loss is caused, videos and audio aredegraded.

(b) Shortage of media rate. When a media rate is in short, media qualityis degraded. For example, videos are fuzzy, grid noise called blocknoise is generated, and audio quality deteriorates.

(c) Remaining reproduction time runs out during content reproduction.When the remaining reproduction time runs out, reproduction of contentis stopped.

(d) Frequent changes of media rate. If the media rate is changedfrequently, media quality varies frequently.

Measures against the grounds (a) to (d) include the followings.

(A) Measures against a packet loss are to prevent the transmission ratefrom becoming too high. As the transmission rate is lower, theprobability of occurrence of congestion can be lower, so that a packetloss can be reduced. As such, it is evaluated that as the transmissionrate is lower, QoE is higher.

(B) Measures against a shortage of media rate are to allow the mediarate to be higher as much as possible. As the media rate is higher,content quality can be higher. As such, it is evaluated that as themedia rate is higher, QoE is higher.

(C) Measures against running out of remaining reproduction time are toprevent the media rate from becoming too high compared with thetransmission rate. As the media rate is lower than the transmissionrate, the case where the remaining reproduction time runs out is lesslikely, whereby it is possible to prevent reproduction of content frombeing discontinued. As such, it is evaluated that as the media rate islower compared with the transmission rate, QoE is higher.

(D) Measures against frequent changes of media rate are to reduce thefrequency of changes of the media rate. As the frequency of changes ofthe media rate is lower, the frequency of changes of the clearness ofthe content can be lower. As such, it is evaluated that as the frequencyof changes is lower, QoE is higher.

Further, according to the state of the client device 200, it is alsopossible to prevent selection of media data which cannot be reproducedby the client device 200. If there is no extra room in the CPU or memoryof the client device 200 or in the network being used, by selectingmedia data not placing a reproduction load on the client device 200, itis possible not to degrade the QoE of the user operating the clientdevice 200.

The QoE evaluation means 102 b evaluates the QoE by using the media rateand the transmission rate as variables and using an objective functionfQ which determines QoE so as to correspond to the variables. Theobjective function fQ for evaluating QoE is constructed of four items z1to z4 relating to the above-described measures (A) to (D). Hereinafter,the objective function fQ will be described. It should be noted that thevalue of the objective function fQ becomes smaller as the QoE is higher.

It is assumed that at a time t, the media rate and the transmission rateof content transmitted from the server device 100 to the client device200 is b(t) and v(t), respectively, and the remaining reproduction timeand the reproduction speed in the client device 200 is T(t) and p(t),respectively. In that case, the items z1 to z4 relating to the measures(A) to (D) are shown by the following expressions.

The item z1, relating to the measures (A) that “measures against apacket loss are to prevent the transmission rate v(t) from becoming toohigh”, corresponds to reducing the value of the objective function fQ asthe transmission rate v(t) is lower. As such, the item z1 is representedby the following Expression (4).z1=v(t)²  (4)

The item z2, relating to the measures (B) that “measures against ashortage of media rate are to allow the media rate b(t) to be higher asmuch as possible”, corresponds to reducing the value of the objectivefunction fQ as the reciprocal of the media rate b(t) is smaller. Assuch, the item z2 is represented by the following Expression (5).z2=1/b(t)²  (5)

The item z3, relating to the measures (C) that “measures against runningout of remaining reproduction time are to prevent the media rate b(t)from becoming too high compared with the transmission rate v(t)”,corresponds to reducing the value of the objective function fQ as thevalue obtained by dividing the media rate b(t) by the transmission ratev(t) is smaller. As such, the item z3 is represented by the followingExpression (6).z3={p(t)/T(t)}·{b(t)/v(t)}²  (6)

Here, p(t)/T(t) is a term for increasing the impact of the item z3 onthe objective function fQ when the reproduction speed p(t) is high andthe remaining reproduction time T(t) is short in the client device 200,while decreasing the impact of the item z3 on the objective function fQwhen the remaining reproduction time T(t) is long and the reproductionspeed p(t) is low.

The item z4, relating to the measures (D) that “measures againstfrequent changes of the media rate b(t) are to reduce the changes of themedia rate b(t)”, corresponds to reducing the value of the objectivefunction fQ as the time rate of change (time-derivative value) of themedia rate b(t) is smaller. As such, the item z4 is represented by thefollowing Expression (7).z4={db(t)/d(t)}²  (7)

In the present embodiment, the objective function fQ, to which therespective items z1 to z4 are weighted and added, is used as theobjective function fQ for evaluating QoE. This objective function isrepresented by the following Expression (8).fQ=a1·z1+a2·z2+a3·z3+a4·z4  (8)

It should be noted that in Expression (8), a1, a2, a3, and a4 arerespectively weights to the items z1, z2, z3, and z4, which can bechanged for each of the types of services and users.

[Evaluation of Delivery Cost]

The delivery cost is evaluated by the delivery cost evaluation means 102c of the rate output means 102.

The user of the client device 200 is free to stop viewing of content,and if the user stops viewing in the middle of the content, the mediadata, which is stored in the media reproduction buffer 202 of the clientdevice 200 and not having been reproduced, is discarded. As such, if theuser stops viewing in the middle of the content, it means that adelivery cost is made for delivering useless media data. On the otherhand, if the remaining reproduction time T(t) in the media reproductionbuffer 202 of the client device 200 is the same, as the media rate b(t)of the non-reproduced media data stored therein is larger, the quantityof stored media data is larger. This means that as the media rate b(t)is larger, the delivery cost is made for delivering larger quantity ofuseless media data. Accordingly, the delivery cost evaluation means 102c evaluates that the delivery cost is larger as the media rate b(t) islarger. More specifically, the delivery cost evaluation means 102 cevaluates the delivery cost using an objective function fC in which themedia rate b(t) is a variable and which determines the delivery cost tocorrespond to the variable, as shown in the following Expression (9). InExpression (9), w1 is a weight. Further, the value of the objectivefunction fC becomes smaller as the delivery cost is smaller.fC=w1·b(t)²  (9)

It should be noted that the above-described objective function fC forevaluating the delivery cost is just an example, and is not limited tothis example. For example, as the objective function fC for evaluatingthe delivery cost, the following Expression (10) may be used.fC=w1·b(t)² +w2·v(t)²  (10)

The first term “w1·b(t)²” on the right-hand side of Expression (10) isthe same as the first term of Expression (9). Further, in the secondterm “w2·v(t)²” on the right-hand side of Expression (10), “w2” is aweight and “v(t)” is a transmission rate when media data is transmittedfrom the server device 100 to the client device 200. The second term“w2·v(t)²” on the right-hand side serves as follows in the objectivefunction fC. The user of the client device 200 is free to stop viewingof the content, and if the user stops viewing in the middle of thecontent, the non-reproduced media data stored in the media reproductionbuffer 202 is discarded. If the media rate b(t) of the non-reproducedmedia data to be discarded is the same, when the remaining reproductiontime T(t) is longer, it means that more useless media data has beentransmitted from the server device 100 to the client device 200. Assuch, it can be evaluated that as the remaining reproduction time T(t)is longer, the delivery cost is higher. Here, the remaining reproductiontime T(t) can be shorter as the transmission rate v(t) is lower.Accordingly, the second term “w2·v(t)²” of the right-hand side acts toreduce the value of the objective function fC (acts to reduce thedelivery cost) as the remaining reproduction time T(t) is shorter (actsto reduce the delivery cost).

Further, as the objective function fC for evaluating the delivery cost,the following Expression (11) can also be used. It should be noted thatthe first term “w2·v(t)²” on the right-hand side of Expression (11) isthe same as the second term on the right-hand side of Expression (10).fC=w2·v(t)²  (11)[Optimum Combination of Media Rate and Transmission Rate]

With consideration of both the QoE and the delivery cost for each clientdevice to which content is to be delivered, the optimum rate calculationmeans 102 a calculates an optimum combination of a media rate b(t) and atransmission rate v(t) for media data to be delivered. Morespecifically, the optimum rate calculation means 102 a calculates, amongcombinations of media rates b(t) and transmission rates v(t) satisfyingpredetermined constraints, a combination of a media rate b(t) and atransmission rate v(t) which minimizes the value of an objectivefunction fT shown by the following Expression (12) (a combination of amedia rate b(t) and a transmission rate v(t) which enables to make QoEas high as possible while reducing the delivery cost as much aspossible).fT=mq·fQ+mc·fC  (12)

In Expression (12), fQ is an objective function for evaluating QoE shownas Expression (8), and fC is any one of the objective functions forevaluating the delivery cost shown as Expressions (9) to (11). Further,mq and mc are weights to the values of the objective functions fQ andfC, respectively. Here, in the objective function fQ for evaluating theQoE, the media rate b(t) and the transmission rate v(t) are variables,while in the objective function fC for evaluating the delivery cost, oneor both of the media rate b(t) and the transmission rate v(t) arevariables. Accordingly, it can be said that the objective function fTshown as Expression (12) is a function in which the media rate b(t) andthe transmission rate v(t) are variables, and which determines anevaluation value evaluating a combination of a delivery cost and QoE isdetermined so as to correspond to the variables. The weights mq and mccan be set freely by the manager or the like of the server device 100.For example, in the case of calculating an optimum combination of amedia rate b(t) and a transmission rate v(t) with a higher emphasis onthe QoE rather than the delivery cost, the weight mq is set to be largerrelative to the weight mc, while in the case of calculating an optimumcombination of a media rate b(t) and a transmission rate v(t) with ahigher emphasis on the delivery cost rather than the QoE, the weight meis set to be higher relative to the weight mq.

Next, constraints used by the optimum rate calculation means 102 a willbe described. In the present embodiment, a first constraint and a secondconstraint are used.

The first constraint is a condition that “a transmission rate v(t) inthe media data transmission means 104 of the server device 100 does notexceed an available bandwidth Th(t)”. Here, as the available bandwidthTh(t), an available bandwidth shown by the reception informationtransmitted from the reception information acquisition means 204 of theclient device 200 is used. By satisfying the first constraint, it ispossible to reliably select a transmission rate not higher than theavailable bandwidth Th(t). As such, a packet loss can be prevented. Itshould be noted that as the first constraint includes the availablebandwidth Th(t) shown by the reception information transmitted from theclient device, it is unique to the client device. For example, in thecase of obtaining a media rate and a transmission rate of a clientdevice C1, a first constraint including the available bandwidthtransmitted from the client device C1 must be used, and in the case ofobtaining a media rate and a transmission rate of a client device C2, afirst constraint including the available band transmitted from theclient device C2 must be used.

The second constraint is a condition that “a media rate b(t) in themedia data transmission means 104 of the server device 100 is any one ofthe predetermined n types of media rates b1, b2, . . . , bn. In thepresent embodiment, in the media data storing means 101, n pieces ofmedia data encoded with the n types of media rates b1, b2, . . . , bnare stored for each piece of content.

It should be noted that the constraints are not limited to thosedescribed above. For example, it is possible to only use either thefirst constraint or the second constraint.

As described above, the optimum rate calculation means 102 a calculates,for each client device to which content is to be delivered, acombination of a media rate b(t) and a transmission rate v(t) satisfyingthe first constraint and the second constraint and minimizing the valueof the objective function fT. Here, while the media data is transmitteddirectly using the calculated media rate b(t) and the transmission ratev(t), in the present embodiment, the media rate b(t) and thetransmission rate v(t) are adjusted based on the states of the serverdevice 100 and the network NW, and media data is transmitted using theadjusted media rate bh(t) and the transmission rate vh(t), as describedbelow. It should be noted that in the case of transmitting media data tothe client device using the media rate b(t) and the transmission ratev(t) before adjustment, it is possible to achieve an advantageous effectof improving the QoE of each user as much as possible while reducing thedelivery cost.

[Adjustment of Media Rate and Transmission Rate]

Next, adjustment processing of a media rate and a transmission rate willbe described. The optimum rate calculation means 102 a adjusts a mediarate h(t) and a transmission rate v(t) for each client device to whichcontent is to be delivered based on the states of the server device 100and the network NW, and outputs the adjusted media rate bh(t) and theadjusted transmission rate vh(t) to the media packet generation means103 and the media data transmission means 104 respectively. Adjustmentbased on the states of the server device 100 and the network NW will bedescribed briefly below. If the CPU load on the server device 100 or thehand of the network NW is tight, the optimum rate calculation means 102a evaluates the delivery cost based on the states of the server device100 and the network NW in such a manner that the delivery cost is highercompared with the case of having extra room relatively. Next, theoptimum rate calculation means 102 a calculates a weight (0≦weight≦1) tothe media rate b(t) and the transmission rate v(t). The value of theweight becomes smaller as the delivery cost is higher. Then, the optimumrate calculation means 102 a multiplies the media rate b(t) and thetransmission rate v(t) by the weight respectively to thereby obtain theadjusted media rate bh(t) and the adjusted transmission rate vh(t).

For example, if the number of units of client devices to which theserver device 100 delivers content simultaneously is Nc [units], the CPUload on the server device 100 is Ls [%], the available memory of theserver device 100 is Ms [bytes], the entire quantity of communicationstransmitted from the server device 100 is Ts [bits/second], the quantityof data transfer involved in reading of media data from the media datastoring means 101 is Td [bytes/second], the delivery cost Cs in theserver device 100 is as shown by the following Expression (13).Cs=k1·Nc+k2·Ls+k3·Ms+k4·Ts+k5·Td  (13)

Here, k1 to k5 are weights to the respective items, and are changeablefor each of the service types and users. The delivery cost Cs shows thatthe delivery cost is higher as the values are larger.

Expression (13) shows that if the number of client devices 200 to whichthe server device 100 delivers content simultaneously is larger, thedelivery cost is higher compared with the case where the number ofclient devices 200 to which the server device 100 delivers contentsimultaneously is smaller. Further, Expression (13) shows that thedelivery cost is higher when the CPU load on the server device 100 ishigh so that the CPU has no extra capacity, when the available memory issmall, when the entire quantity of communications from the server device100 is large so that the communicable quantity is tight, when thequantity of data transfer involved in reading of media data is large sothat data transfer is tight, and the like, compared with the case havingextra capacity.

Further, when the entire network NW is in congestion (when the trafficvolume is large), the optimum rate calculation means 102 a has prepareda function which calculates a delivery cost larger than the case wherethe network is available, and by using the function, calculates thedelivery cost Cn in the network NW. Then, by adding the delivery cost Csand the delivery cost Cn, the optimum rate calculation means 102 acalculates a delivery cost Ct=(Cs+Cn) corresponding to the states of theserver device 100 and the network NW. It should be noted that if thecongestion degree of the entire network NW is unknown, it is alsopossible to prepare a function which calculates different delivery costsdepending on time zones, and using the delivery cost calculated by usingthe function as the delivery cost Cn in the network NW. For example, itis acceptable that as the network NW is not congested in the midnight, asmaller delivery cost is calculated, while as a larger number of peopleuse the network NW from 21:00 to 22:00 at night, the delivery cost Cn ofthe entire network NW is calculated using a function which calculates ahigher delivery cost Cn. Further, it is also possible to change thedelivery cost depending on the location on the network where packets runthrough, rather than the congestion degree of the entire network. Forexample, as the delivery cost with respect to the client device 200passing through the network link where the communication traffic isconcentrated so that it may easily becomes a bottleneck link, a valuecalculated by multiplying the delivery cost Ct by a weight larger than“1” may be used.

It is also possible to multiply the delivery cost Ct by a weightcorresponding to the service level of the user of the client device. Forexample, it is possible to multiply the delivery cost Ct of the freemembers by a weight larger than that multiplied to the delivery cost Ctof the paying members so that the paying members can view higher-qualitycontent compared with the free members. Further, it is also possible tomultiply the delivery cost Ct of the users of lower service levels by aweight larger than that multiplied to the delivery cost Ct of the usersof higher service levels, or multiply a weight which is inverselyproportional to the service fees.

When the optimum rate calculation means 102 a calculates a delivery costCt, the optimum rate calculation means 102 a calculates a weight W withrespect to the media rate b(t) and the transmission rate v(t) using afunction fAC(Ct) which monotonically decreases from 1 to 0.

Then, the optimum rate calculation means 102 a performs operations shownin the following Expressions (14) and (15) to thereby calculate theadjusted media rate bh(t) and the adjusted transmission rate vh(t).bh(t)=W·b(t)  (14)vh(t)=W·v(t)  (15)

Then, the optimum rate calculation means 102 a provides the media packetgeneration means 103 and the media data transmission means 104 with theadjusted media rate bh(t) and the adjusted transmission rate vh(t),respectively. The optimum rate calculation means 102 a performs theabove-described processing for each client device to which content is tobe delivered. The details of the processing performed at step S002 areas described above. It should be noted that the method of calculatingthe delivery cost Ct and the method of calculating the weight using thefunction fAC(Ct) as described above are just examples, and are notlimited to them.

When the media packet generation means 103 receives the media rate bh(t)from the optimum rate calculation means 102 a, the media packetgeneration means 103 inputs media data encoded with the media rate bh(t)from the media data storing means 101 in which a plurality of pieces ofmedia data encoded with different media rates are stored. Then, themedia packet generation means 103 generates media packets based on theinput media data, and provides the media data transmission means 104with the generated media packets. It should be noted that media packetgeneration means 103 may transcode the media data input from the mediadata storing means 101 into media data of the media rate received fromthe optimum rate calculation means 102 a. The media data transmissionmeans 104 transmits the media packets provided by the media packetgeneration means 103 to the corresponding client device (e.g., clientdevice 200) at the transmission rate vh(t) provided by the optimum ratecalculation means 102 a (step S003).

Regarding transmission of the media data at the calculated transmissionrate, if it is transmitted using UDP/IP, it is only necessary totransmit the media packets in appropriate timing. For example, if mediadata is made into packets of 1500 bytes and transmitted at 300 kbps, asit is possible to transmit (300*1000)/(1500*8)=25 packets per second,transmission is controlled to transmit one packet per 40 milliseconds.

If transmission is performed using TCP/IP, as a packet transmission timecannot be controlled, the time to transmit the next packet is controlledbased on the time taken for transmitting the previous packet. In thecase of transmitting packets of 1500 bytes at 300 kbps, 1500 bytes aretransmitted every 40 milliseconds. If 30 milliseconds have been takenfor transmitting a packet of 1500 bytes, it is only necessary to controltransmission such that the next packet of 1500 bytes is transmittedafter waiting for 10 milliseconds (=40 milliseconds 30 milliseconds).

The above-described processing is repeated until the media data ends(step S004 in FIG. 3).

Effects of First Exemplary Embodiment

According to the present embodiment, it is possible to achieve anadvantageous effect of improving QoE as much as possible while reducingthe delivery cost. This is because the present embodiment is adapted tocalculate a media rate and a transmission rate when delivering contentby using an objective function which uses the media rate and thetransmission rate of media data as variables and determines anevaluation value for evaluating a combination of QoE and a delivery costso as to correspond to the variables.

Further, according to the present embodiment, it is possible to reducethe delivery cost while equally improving the QoE of the entire usersutilizing the content delivery system. This is because the presentembodiment is adapted to adjust the values of a media rate and atransmission rate calculated using an objective function such that thevalues become smaller as the load on the server device and the networkcongestion are tighter, and deliver media data using the adjusted mediarate and the adjusted transmission rate. As such, it is possible toprevent a situation where particular users use a large number ofresources.

Second Exemplary Embodiment

Next, a second exemplary embodiment of the present invention will bedescribed in detail with reference to FIG. 4.

The present embodiment is different from the content delivery system ofthe first exemplary embodiment in that a client device 400 includes arate output means 407, and selects an optimum combination of a mediarate and a transmission rate in the client device 400. Hereinafter,description will be given below focusing on such a difference.

The content delivery system according to the second exemplary embodimentincludes a server device 300 and the client device 400. The serverdevice 300 and the client device 400 are connected communicably witheach other over a network.

The server device 300 includes a media data storing means 301, a mediapacket generation means 302, and a media data transmission means 303.

Further, the client device 400 includes a media data reception means401, a media reproduction buffer 402, a media data reproduction means403, a reception information acquisition means 404, a reproduction speedacquisition means 405, a remaining reproduction time acquisition means406, a rate output means 407, and a transmission request means 408.Further, the rate output means 407 includes an optimum rate calculationmeans 407 a, a QoE evaluation means 407 b, and a delivery costevaluation means 407 c.

The media data storing means 301, the media packet generation means 302,and the media data transmission means 303 of the server device 300 havefunctions similar to those of the media data storing means 101, themedia packet generation means 103, and the media data transmission means104 of the first exemplary embodiment, respectively.

The means 401 to 408 of the client device 400 operate as follows,respectively.

The media data reception means 401, the media reproduction buffer 402,the media data reproduction means 403, the reception informationacquisition means 404, the reproduction speed acquisition means 405, theremaining reproduction time acquisition means 406, the rate output means407, the optimum rate calculation means 407 a, the QoE evaluation means407 b, and the delivery cost evaluation means 407 c have functionssimilar to those of the media data reception means 201, the mediareproduction buffer 202, the media data reproduction means 203, thereception information acquisition means 204, the reproduction speedacquisition means 205, the remaining reproduction time acquisition means206, the rate output means 102, the optimum rate calculation means 102a, the QoE evaluation means 102 b, and the delivery cost evaluationmeans 102 c of the first exemplary embodiment, respectively.

The transmission request means 408 requests the server device 300 todeliver media data of a media rate calculated by the rate output means407, at a transmission rate calculated by the rate output means 407.

The media packet generation means 302 of the server device 300 reads themedia data of the media rate, requested by the client device 400, fromthe media data storing means 301, or transforms the media rate of themedia data read from the media data storing means 301 into the requestedmedia rate. Then, based on the readout or transformed media data, themedia packet generation means 302 generates media packets, and providesthe media data transmission means 303 with the generated media packets.

The media data transmission means 303 transmits the media packets,provided by the media packet generation means 302, to the client device400 at the transmission rate requested by the client device 400.

Effects of Second Exemplary Embodiment

According to the present embodiment, it is possible to achieve anadvantageous effect that the load on the server device 300 is reduced,in addition to the advantageous effects achieved in the first exemplaryembodiment. This is because the rate output means 407 is provided to theclient device 400.

Third Exemplary Embodiment

Next, a third exemplary embodiment of the present invention will bedescribed with reference to the drawings.

A content delivery system of the third exemplary embodiment is differentfrom the content delivery system of the first exemplary embodiment inthat a relay device, which transfers media data from a server device toa client device, acquires reception information, and transmits theacquired reception information to the server device.

As shown in FIG. 5, the content delivery system of the presentembodiment includes a server device 500, a client device 600, and arelay device 700. The server device 500 and the relay device 700 arecommunicably connected with each other over a first network NW1, and therelay device 700 and the client device 600 are communicably connectedwith each other over a second network NW2.

In an example of the present embodiment, the client device 600 may be amobile telephone, a smartphone, or a tablet connected to a mobiletelephone network, and the relay device 700 may be a base stationdevice. In that case, the first network NW1 may be the Internet or acore network of a telecom operator, and the second network NW2 may be awireless network for mobile telephones.

In another example, the client device 600 may be a PC, a TV receiver, ora set top box connected within home, and the relay device 700 may be ahome gateway or a broadband router. Further, the first network NW1 maybe the Internet or an access network or a core network of a telecomoperator, and the second network NW2 may be a wired or wireless networkwithin home. These are just examples, without any limitation.

The relay device 700 includes a central processing unit (CPU) andstoring units (memory and hard disk drive (HDD)). The relay device 700is adapted to realize functions described below by means of execution ofa program, stored in the storing units, by the CPU.

FIG. 6 is a block diagram showing functions of the delivery systemconfigured as described above. The present embodiment is different fromthe content delivery system of the first exemplary embodiment in thatthe relay device 700 is added. Hereinafter, description will be givenbelow focusing on such a difference.

The server device 500 includes a media data storing means 501, a rateoutput means 502, a media packet generation means 503, and a media datatransmission means 504. The rate output means 502 includes an optimumrate calculation means 502 a, a QoE evaluation means 502 b, and adelivery cost evaluation means 502 c.

The media data storing means 501, the rate output means 502, the mediapacket generation means 503, the media data transmission means 504, theoptimum rate calculation means 502 a, the QoE evaluation means 502 b,and the delivery cost evaluation means 502 c of the server device 500have functions similar to those of the media data storing means 101, therate output means 102, the media packet generation means 103, the mediadata transmission means 104, the optimum rate calculation means 102 a,the QoE evaluation means 102 b, and the delivery cost evaluation means102 c of the first exemplary embodiment, respectively.

Further, the client device 600 includes a media data reception means601, a media reproduction buffer 602, a media data reproduction means603, a reproduction speed acquisition means 604, and a remainingreproduction time acquisition means 605.

The media data reception means 601, the media reproduction buffer 602,the media data reproduction means 603, the reproduction speedacquisition means 604, and the remaining reproduction time acquisitionmeans 605 of the client device 600 have functions similar to those ofthe media data reception means 201, the media reproduction buffer 202,the media data reproduction means 203, the reproduction speedacquisition means 205, and the remaining reproduction time acquisitionmeans 206 of the first exemplary embodiment, respectively.

The relay device 700 includes a data transfer means 701 and a receptioninformation acquisition means 702.

The means 701 and 702 of the relay device 700 operate as follows,respectively.

The data transfer means 701 transfers data for the client device 600,transmitted from the server device 500, to the client device 600, andtransfers data for the server device 500, transmitted from client device600, to the server device 500. The data transmitted from the serverdevice 500 and transferred to the client device 600 includes media data.The data transmitted from the client device 600 and transferred to theserver device 500 includes data of a reproduction speed, a remainingreproduction time, and others such as a client state.

The reception information acquisition means 702 transmits, to the serverdevice 500, a value calculated by dividing the quantity of media data,sequentially transmitted by the data transfer means 701 from the serverdevice 500 to the client device 600, by the transfer time period of themedia data, as reception information of the client device 600. Forexample, if the data transfer means 701 sequentially transfers mediadata of “0.5 megabytes” in “0.5 seconds” from the server device 500 tothe client device 600, and then it cannot receive the media data fromthe server device 500 for a given period and cannot execute transferprocessing, reception information is 0.5 megabytes/0.5 seconds=1(megabytes/second). It should be noted that acquisition processing ofreception information is performed every predetermined time. While it isassumed that transfer of the media data is performed using TCP/IP, it isalso possible to use UDP/IP rather than TCP/IP. In the case of usingUDP/IP, there is a possibility that media packets transmitted from theserver device 500 are lost and cannot be received by the media datareception means 601. In that case, a rate of loss of media packets (rateof occurrence of packet loss) lost between the server device 500 and therelay device 700, and a rate of loss of media packets lost between therelay device 700 and the client device 600, are also acquired. Forexample, in the case of using RTP, a rate of packet loss is calculatedby specifying the lost packets based on the sequence number included inthe RTP header. If the network NW2, to which the relay device 700 isconnected, is a mobile telephone network and it is possible to measurethe state of radio wave signals between it and the client device 600, itis acceptable to acquire a state such as a state of radio wave signalsbetween it and the client device 600. The reception informationacquisition means 702 transfers the acquired reception information tothe server device 500.

Effects of Third Exemplary Embodiment

According to the present embodiment, it is possible to achieve anadvantageous effect that the configuration of a client device can besimplified, in addition to the advantageous effects achieved by thefirst exemplary embodiment, because the reception informationacquisition means 702 is provided to the relay device 700.

Fourth Exemplary Embodiment

Next, a fourth exemplary embodiment of the present invention will bedescribed with reference to the drawings.

Referring to FIG. 7, a content delivery system of the present embodimentincludes a server device 800 and a client device 900 which arecommunicably connected with each other. The server device 800 is adaptedto be able to transmit media data, formed by encoding content, to theclient device 900. The client device 900 is adapted to, while receivingmedia data transmitted from the server device 800, store received dataof the media data in a media reproduction buffer, read stored media datastore from the media reproduction buffer, and reproduce the content.

Further, the content delivery system of the present embodiment alsoincludes a rate output means 801 and a media data transmission means802.

The rate output means 801 calculates and outputs a combination of atransmission rate and a media rate based on an objective function. Theobjective function is a function which uses a transmission rate,representing the quantity of media data to be transmitted per unit timeby the server device 800, and a media rate, representing the quantity ofmedia data per unit time, as variables, and determines an evaluationvalue evaluating a combination of QoE, which is a service quality that auser of the client device 900 experiences when viewing the contentreproduced by the client device 900, and a delivery cost representingthe cost involved in delivering the media data so as to correspond tothe variables.

Further, the media data transmission means 802 transmits the media data,encoded at the media rate output from the rate output means 801, fromthe server device 800 to the client device 900 at the transmission rateoutput from the rate output means 801.

Effects of Fourth Exemplary Embodiment

According to the present embodiment, it is possible to achieve anadvantageous effect of improving QoE as much as possible while reducingthe delivery cost. This is because a media rate and a transmission rate,when delivering content, is calculated using an objective function whichuses the media rate and the transmission rate of the media data asvariables and determines an evaluation value evaluating a combination ofQoE and the delivery cost so as to correspond to the variables.

The whole or part of the exemplary embodiments disclosed above can bedescribed as, but not limited to, the following supplementary notes.

(Supplementary Note 1)

A content delivery system comprising a server device and a client devicewhich are adapted to be communicable with each other, wherein

the server device is adapted to be able to transmit media data, formedby encoding content, to the client device,

the client device is adapted to, while receiving the media datatransmitted by the server device, store received data of the media datain a media reproduction buffer, read stored media data from the mediareproduction buffer, and reproduce the content, and

the content delivery system further comprises:

-   -   rate output means for calculating and outputting a combination        of a transmission rate and a media rate based on an objective        function, the transmission rate representing a quantity of the        media data transmitted per unit time by the server device, the        media rate representing a quantity of the media data per unit        time, the objective function using the transmission rate and the        media rate as variables and determining an evaluation value        evaluating a combination of QoE (Quality of Experience), which        is a service quality that a user of the client device        experiences when viewing the content reproduced by the client        device, and a delivery cost representing a cost involved in        delivering the media data so as to correspond to the variables;        and    -   media data transmission means for transmitting the media data,        encoded at the media rate output from the rate output means,        from the server device to the client device at the transmission        rate output from the rate output means.        (Supplementary Note 2)

The content delivery system, according to supplementary note 1, wherein

the objective function is set such that the delivery cost becomes higheras the media rate is higher.

(Supplementary Note 3)

The content delivery system, according to supplementary note 1 or 2,wherein

the objective function is set such that the delivery cost becomes loweras the transmission rate is lower.

(Supplementary Note 4)

The content delivery system, according to any of supplementary notes 1to 3, wherein

the objective function is set such that the delivery cost becomes higheras a remaining reproduction time is longer, the remaining reproductiontime being a period of time in which the content is able to bereproduced by the media data stored in the media reproduction buffer andnot having been reproduced.

(Supplementary Note 5)

The content delivery system, according to any of supplementary notes 1to 4, wherein

the rate output means adjusts the calculated transmission rate and thecalculated media rate such that values of the calculated transmissionrate and the calculated media rate become smaller as a load placed onthe server device is higher, and outputs the adjusted transmission rateand the adjusted media rate.

(Supplementary Note 6)

The content delivery system, according to any of supplementary notes 1to 5, wherein

the rate output means adjusts the calculated transmission rate and thecalculated media rate such that values of the calculated transmissionrate and the calculated media rate become smaller as a congestion degreeof a network connecting the server device and the client device ishigher, and outputs the adjusted transmission rate and the adjustedmedia rate.

(Supplementary Note 7)

The content delivery system, according to any of supplementary notes 1to 6, wherein

the rate output means adjusts the calculated transmission rate and thecalculated media rate such that values of the calculated transmissionrate and the calculated media rate become smaller in a time zone duringwhich a congestion degree of a network connecting the server device andthe client device is expected to be higher, and outputs the adjustedtransmission rate and the adjusted media rate.

(Supplementary Note 8)

The content delivery system, according to any of supplementary notes 1to 7, wherein

the rate output means adjusts the calculated transmission rate and thecalculated media rate such that values of the calculated transmissionrate and the calculated media rate become smaller as a service level ofthe user of the client device is lower, and outputs the adjustedtransmission rate and the adjusted media rate.

(Supplementary Note 9)

The content delivery system, according to any of supplementary notes 1to 8, wherein

the objective function is set such that the QoE becomes higher as thetransmission rate is lower.

(Supplementary Note 10)

The content delivery system, according to any of supplementary notes 1to 9, wherein

the objective function is set such that the QoE becomes higher as themedia rate is higher.

(Supplementary Note 11)

The content delivery system, according to any of supplementary notes 1to 10, wherein

the objective function is set such that the QoE becomes higher asfrequency of changes of the media rate is lower.

(Supplementary Note 12)

The content delivery system, according to any of supplementary notes 1to 11, wherein

the objective function is set such that the QoE becomes higher as aremaining reproduction time is longer, the remaining reproduction timebeing a period of time in which the content is able to be reproduced bythe media data stored in the media reproduction buffer and not havingbeen reproduced.

(Supplementary Note 13)

The content delivery system, according to any of supplementary notes 1to 12, wherein

the rate output means is adapted to output, among combinations of thetransmission rates and the media rates satisfying predeterminedconstraints, a combination of a transmission rate and a media rate withwhich the evaluation value, corresponding thereto by the objectivefunction, is the highest.

(Supplementary Note 14)

A server device adapted to be communicable with a client device and tobe able to transmit media data, formed by encoding content, to theclient device, the server device comprising:

rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables; and

media data transmission means for transmitting the media data, encodedat the media rate output from the rate output means, from the serverdevice to the client device at the transmission rate output from therate output means.

(Supplementary Note 15)

The server device, according to supplementary note 14, wherein

the objective function is set such that the delivery cost becomes higheras the media rate is higher.

(Supplementary Note 16)

The server device, according to supplementary note 14 or 15, wherein

the objective function is set such that the delivery cost becomes loweras the transmission rate is lower.

(Supplementary Note 17)

A client device adapted to be communicable with a server device and,while receiving media data transmitted by the server device, storereceived data of the media data in a media reproduction buffer, readstored media data from the media reproduction buffer, and reproducecontent, the client device comprising

rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables.

(Supplementary Note 18)

The client device, according to supplementary note 17, wherein

the objective function is set such that the delivery cost becomes higheras the media rate is higher.

(Supplementary Note 19)

The client device, according to supplementary note 17 or 18, wherein

the objective function is set such that the delivery cost becomes loweras the transmission rate is lower.

(Supplementary Note 20)

A delivery method to be applied to a content delivery system including aserver device and a client device which are adapted to be communicablewith each other, the server device being adapted to be able to transmitmedia data, formed by encoding content, to the client device, the clientdevice being adapted to, while receiving the media data transmitted bythe server device, store received data of the media data in a mediareproduction buffer, read stored media data from the media reproductionbuffer, and reproduce the content, the method comprising:

calculating and outputting a combination of a transmission rate and amedia rate based on an objective function, the transmission raterepresenting a quantity of the media data transmitted per unit time bythe server device, the media rate representing a quantity of the mediadata per unit time, the objective function using the transmission rateand the media rate as variables and determining an evaluation valueevaluating a combination of QoE (Quality of Experience), which is aservice quality that a user of the client device experiences whenviewing the content reproduced by the client device, and a delivery costrepresenting a cost involved in delivering the media data so as tocorrespond to the variables; and

transmitting the media data, encoded at the output media rate, from theserver device to the client device at the output transmission rate.

(Supplementary Note 21)

The delivery method, according to supplementary note 20, wherein

the objective function is set such that the delivery cost becomes higheras the media rate is higher.

(Supplementary Note 22)

The delivery method, according to supplementary note 20 or 21, wherein

the objective function is set such that the delivery cost becomes loweras the transmission rate is lower.

(Supplementary Note 23)

A computer-readable medium storing a program for causing a computer tofunction as a server device, the program causing the computer tofunction as:

rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of a client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables; and

media data transmission means for transmitting the media data, encodedat the media rate output from the rate output means, from the serverdevice to the client device at the transmission rate output from therate output means.

(Supplementary Note 24)

A computer-readable medium storing a program for causing a computer tofunction as a client device, the program causing the computer tofunction as:

reproduction means for, while receiving media data transmitted by aserver device, storing received data of the media data in a mediareproduction buffer, reading stored media data from the mediareproduction buffer, and reproducing content; and

rate output means for calculating and outputting a combination of atransmission rate and a media rate based on an objective function, thetransmission rate representing a quantity of the media data transmittedper unit time by the server device, the media rate representing aquantity of the media data per unit time, the objective function usingthe transmission rate and the media rate as variables and determining anevaluation value evaluating a combination of QoE (Quality ofExperience), which is a service quality that a user of a client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables.

While the present invention has been described with reference to theexemplary embodiments described above, the present invention is notlimited to the above-described embodiments. The form and details of thepresent invention can be changed within the scope of the presentinvention in various manners that can be understood by those skilled inthe art.

Further, while the programs of the above-described embodiments arestored in storing units, they may be stored in computer-readablerecording media. For example, recording media are portable mediaincluding flexible disks, optical disks, magneto-optical disks, andsemiconductor memories.

INDUSTRIAL APPLICABILITY

The present invention is applicable to, for example, a content deliverysystem including a server device which transmits media data representingcontent and a client device which receives media data from the serverdevice and reproduces it.

REFERENCE NUMERALS

-   100 server device-   101 media data storing means-   102 rate output means-   102 a optimum rate calculation means-   102 b QoE evaluation means-   102 c delivery cost evaluation means-   103 media packet generation means-   104 media data transmission means-   200 client device-   201 media data reception means-   202 media reproduction buffer-   203 media data reproduction means-   204 reception information acquisition means-   205 reproduction speed acquisition means-   206 remaining reproduction time acquisition means-   300 server device-   301 media data storing means-   302 media packet generation means-   303 media data transmission means-   400 client device-   401 media data reception means-   402 media reproduction buffer-   403 media data reproduction means-   404 reception information acquisition means-   405 reproduction speed acquisition means-   406 remaining reproduction time acquisition means-   407 rate output means-   407 a optimum rate calculation means-   407 b QoE evaluation means-   407 c delivery cost evaluation means-   408 transmission request means-   500 server device-   501 media data storing means-   502 rate output means-   502 a optimum rate calculation means-   502 b QoE evaluation means-   502 c delivery cost evaluation means-   503 media packet generation means-   504 media data transmission means-   600 client device-   601 media data reception means-   602 media reproduction buffer-   603 media data reproduction means-   604 reproduction speed acquisition means-   605 remaining reproduction time acquisition means-   700 relay device-   701 data transfer means-   702 reception information acquisition means-   800 server device-   801 rate output means-   802 media data transmission means-   900 client device

The invention claimed is:
 1. A content delivery system comprising aserver device and a client device which are adapted to be communicablewith each other, wherein the server device is configured to transmitmedia data, formed by encoding content, to the client device, the clientdevice is configured to, while receiving the media data transmitted bythe server device, store received data of the media data in a mediareproduction buffer, read stored media data from the media reproductionbuffer, and reproduce the content, and the content delivery systemfurther comprises: a rate output unit that calculates and outputs acombination of a transmission rate and a media rate based on a thirdobjective function that is obtained by weighting addition of a firstobjective function and a second objective function, the transmissionrate representing a quantity of the media data transmitted per unit timeby the server device, the media rate representing a quantity of themedia data per unit time, the first objective function using thetransmission rate and the media rate as variables, the second objectivefunction using one or both of the transmission rate and the media rateas variables or a variable, and the third objective function determiningan evaluation value evaluating a combination of Quality of Experience(QoE), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables; and a media data transmissionunit that transmits the media data, encoded at the media rate outputtedfrom the rate output unit, from the server device to the client deviceat the transmission rate outputted from the rate output unit.
 2. Thecontent delivery system, according to claim 1, wherein the secondobjective function is set such that the delivery cost becomes higher asthe media rate is higher.
 3. The content delivery system, according toclaim 1, wherein the second objective function is set such that thedelivery cost becomes lower as the transmission rate is lower.
 4. Thecontent delivery system, according to claim 1, wherein the secondobjective function is set such that the delivery cost becomes higher asa remaining reproduction time is longer, the remaining reproduction timebeing a period of time in which the content is able to be reproduced bythe media data stored in the media reproduction buffer and not havingbeen reproduced.
 5. The content delivery system, according to claim 1,wherein the rate output unit adjusts the calculated transmission rateand the calculated media rate such that values of the calculatedtransmission rate and the calculated media rate become smaller as a loadplaced on the server device is higher, and outputs the adjustedtransmission rate and the adjusted media rate.
 6. The content deliverysystem, according to claim 1, wherein the rate output unit adjusts thecalculated transmission rate and the calculated media rate such thatvalues of the calculated transmission rate and the calculated media ratebecome smaller as a congestion degree of a network connecting the serverdevice and the client device is higher, and outputs the adjustedtransmission rate and the adjusted media rate.
 7. The content deliverysystem, according to claim 1, wherein the rate output unit adjusts thecalculated transmission rate and the calculated media rate such thatvalues of the calculated transmission rate and the calculated media ratebecome smaller in a time zone during which a congestion degree of anetwork connecting the server device and the client device is expectedto be higher, and outputs the adjusted transmission rate and theadjusted media rate.
 8. A server device adapted to be communicable witha client device and to be able to transmit media data, formed byencoding content, to the client device, the server device comprising: arate output unit that calculates and outputs a combination of atransmission rate and a media rate based on a third objective functionthat is obtained by weighting addition of a first objective function anda second objective function, the transmission rate representing aquantity of the media data transmitted per unit time by the serverdevice, the media rate representing a quantity of the media data perunit time, the first objective function using the transmission rate andthe media rate as variables, the second objective function using one orboth of the transmission rate and the media rate as variables or avariable, and the third objective function determining an evaluationvalue evaluating a combination of Quality of Experience (QoE), which isa service quality that a user of the client device experiences whenviewing the content reproduced by the client device, and a delivery costrepresenting a cost involved in delivering the media data so as tocorrespond to the variables; and a media data transmission unit thattransmits the media data, encoded at the media rate outputted from therate output unit, from the server device to the client device at thetransmission rate outputted from the rate output unit.
 9. A clientdevice adapted to be communicable with a server device and, whilereceiving media data transmitted by the server device, store receiveddata of the media data in a media reproduction buffer, read stored mediadata from the media reproduction buffer, and reproduce content, theclient device comprising a rate output unit that calculates and outputsa combination of a transmission rate and a media rate based on a thirdobjective function that is obtained by weighting addition of a firstobjective function and a second objective function, the transmissionrate representing a quantity of the media data transmitted per unit timeby the server device, the media rate representing a quantity of themedia data per unit time, the first objective function using thetransmission rate and the media rate as variables, the second objectivefunction using one or both of the transmission rate and the media rateas variables or a variable, and the third objective function determiningan evaluation value evaluating a combination of Quality of Experience(QoE), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables.
 10. A delivery method to beapplied to a content delivery system including a server device and aclient device which are adapted to be communicable with each other, theserver device being adapted to be able to transmit media data, formed byencoding content, to the client device, the client device being adaptedto, while receiving the media data transmitted by the server device,store received data of the media data in a media reproduction buffer,read stored media data from the media reproduction buffer, and reproducethe content, the method comprising: calculating and outputting acombination of a transmission rate and a media rate based on a thirdobjective function that is obtained by weighting addition of a firstobjective function and a second objective function, the transmissionrate representing a quantity of the media data transmitted per unit timeby the server device, the media rate representing a quantity of themedia data per unit time, the first objective function using thetransmission rate and the media rate as variables, the second objectivefunction using one or both of the transmission rate and the media rateas variables or a variable, and the third objective function determiningan evaluation value evaluating a combination of Quality of Experience(QoE), which is a service quality that a user of the client deviceexperiences when viewing the content reproduced by the client device,and a delivery cost representing a cost involved in delivering the mediadata so as to correspond to the variables; and transmitting the mediadata, encoded at the output media rate, from the server device to theclient device at the output transmission rate.